This paper focuses on the combustion system development and
combustion analysis results for a normally aspirated 0.43-liter
small engine. The inline two-cylinder engine used in experiments
has been tested in a variety of normally aspirated modes, using
98-RON pump gasoline. Test modes were defined by alterations to the
induction system, which included carburetion and port fuel
injection fuel delivery systems. The results from this paper
provide some insight into the combustion effects for small cylinder
normally aspirated spark ignition engines. This information
provides future direction for the development of smaller engines as
oil prices fluctuate and CO₂ emissions begin to be regulated.
Small engine combustion is explored with a number of parametric
studies, including a range of manifold absolute pressures up to
wide open throttle, engine speeds exceeding 10,000 rev/min and
compression ratios ranging from 9 to 13. Combustion system
optimization through compression ratio development enabled the
engine to achieve 37% brake thermal efficiency and 13 bar brake
mean effective pressure. Hence, the test engine performance and
efficiency results demonstrate that smaller bore engines can match
or exceed typical larger bore engines found in passenger vehicles.
However, this was only possible after compression ratio
optimization to compensate for the higher levels of dissociation,
friction and heat losses associated with the small cylinder
size.